This invention relates to a system and method for the transport of voice or data traffic over a telecommunications network, and in particular to collision avoidance in such a network.
A recent innovation in telecommunications technology has been the introduction of the asynchronous transfer mode (ATM) transmission protocol. In this technique, information to be transmitted is loaded into packets or cells each of which is provided with a header containing information that is used for routing the cell to its correct destination. The header of the packet or ATM cell may also contain other protocol control information relating to the cell payload and, in particular may include a cell sequence number. The sequence number can be used to indicate missing cells, or cells that have been unacceptably delayed.
Before a cell stream representing a connection is transmitted between end points in an ATM network, it is necessary to set up a virtual channel connection (VCC) that will carry the traffic. For switched virtual connections (SVCs), this is effected by an ATM signaling procedure, whereas for permanent virtual connections (PVCs) it is effected by a management procedure. Once an ATM connection is available, it may be used by any of the standard ATM adaptation layers, including AAL2, to carry traffic. AAL2 enables multiple calls to be transported on a single ATM VCC via a multiplexing technique that places short packets of information (voice or data) within an ATM cell. Individual AAL2 calls on the same VCC are distinguished by their Channel Identifier (CID) values, carried within the headers of the short information packets. Up to 248 CID values are available for use on a VCC. It will be appreciated that, in order to prevent collision between calls or connections, the same CID should not be allocated on the same VCC to more than one connection at any one time. However, it is possible for two end points or nodes to independently allocate the same CID simultaneously thus leading to a collision situation. This of course can reduce the quality of service that can be provided by the network.
There are various techniques available for reducing the risk of collision, but these tend to fail under high traffic load conditions. This can lead to increased queuing and, under extreme conditions, to an effective breakdown of the network.
An object of the invention is to minimise or to overcome the above disadvantage.
A further object of the invention is to provide an improved arrangement and method for preventing and/or resolving connection collision in an asynchronous network.
A further object of the invention is to provide an improved network node construction.
According to a first aspect of the invention, there is provided method of setting up a connection over an ATM virtual channel connection (VCC) between first and second nodes in an asynchronous network in which said connections are identified by respective connection identifier (CID) values, the method comprising assigning a said CID value to each connection request, and, in the event of simultaneous assignment of the same CID value by both said nodes, awarding that connection to a predetermined one of said nodes.
According to another aspect of the invention, there is provided a method of setting up a connection over an ATM virtual channel connection (VCC) between first and second nodes in an asynchronous network in which said AAL2 connections are identified by respective connection identifier (CID) values, the method comprising;
at the first node, assigning a CID value to a connection request, storing the assigned CID value in a first status record associated with the first node, and signalling that connection request and assigned CID value over a signalling channel to the second node;
at the second node, receiving the connection request and CID value, comparing the CID value with a stored set of assigned CID values in a second status record associated with the second node, where the CID value is not assigned in the second status record, signalling to the first node a message accepting the connection request, and, where the CID value is assigned in the second status record, signalling to the first node a message refusing the connection request
According to a further aspect of the invention, there is provided an asynchronous network node arranged for setting up connections over an ATM virtual channel connection (VCC) between that node and a similar further node in an asynchronous network in which the connections are identified by respective connection identifier (CID) values, the network node comprising; means for assigning a first CID value to a first connection request, storage means comprising status record for storing that assigned CID value together with other CID values assigned by the assigning means, means for signalling that connection request and CID value over a signalling channel to the further node; means for receiving from the further node a further connection request and an associated further CID value, means for comparing the further CID value with the stored assigned CID values, and means for signalling to the further node, where the CID value is not marked as being assigned in the status store, a first message accepting the connection request or, where the CID value is marked as being assigned in the status store, a second message refusing the connection request.
According to another aspect of the invention, there is provided a method of setting up connections over an ATM virtual channel connection (VCC) between first and second nodes in an asynchronous network in which said connections are identified by respective connection identifier (CID) values, the method comprising allocating sets of said CID values to one of said nodes which node becomes the owner of those CID values, transmitting connection requests at each said node and attaching a respective CID value to each said request, and, in the event of a potential collision arising from the simultaneous allocation of the same CID value by both nodes, accepting the connection request from said one node owning the CID value and refusing the connection request from the other node.
According to a further aspect of the invention, there is provided an arrangement for setting up connections in response to connection requests over ATM virtual channel connection (VCC) between first and second nodes in an asynchronous network in which connections are identified by respective connection identifier (CID) values, the arrangement comprising; at each said node,
means for assigning a CID value to a said connection request, means for storing the assigned CID value in a status record, means for signalling that connection request and assigned CID value over a signalling channel to the other said node; means for receiving a further connection request and CID value from the other said node, means for comparing the further CID value with the stored assigned CID values in the status record, and means for signalling to the said other node a first message accepting the connection request where the further CID value is not marked as being assigned in the status record or, where the further CID value is marked as being assigned in the status record, signalling to the further node a second message refusing the connection request.
As each node checks the CID of each received connection request against its own status record of assigned requests, a connection can be refused where potential duplicate allocation of the same CID is discovered. Thus, connections are refused before potential collisions can occur.
Typically, the connections are ATM Adaptation Layer Two (AAL2) connections.
In a preferred embodiment, CID values are assigned to one node so that, in the event of a potential collision, resolution is effected by seizure of the connection by that one node.